Celtics' Shaquille O'Neal, center, limps off the court after being injured against the Detroit Pistons April 3.AP

WALTHAM, Mass. — Shaquille O’Neal is out tomorrow, probably longer.

The Celtics’ center, who has been trying to come back from a calf injury, failed in a game-simulation situation today and was ruled out for Game 1 of the first round series with the Knicks tomorrow at least — and likely Game 2, possibly the entire series — by president of basketball operations Danny Ainge and team physician Dr. Brian McKeon.

“He’s been making progress,” Ainge said before the end of today’s practice here. “Shaq, we tried to get him out there in practice today to simulate a game as much as we could and he’s not ready. So we don’t know when he’ll be ready. We’ll just keep evaluating him day to day. So he won’t play in tomorrow’s game.”

McKeon, who labeled Shaq the “toughest” athlete he has treated, said O’Neal tried his simulation — which was basically running — after several hours of treatment.

“He’s been working his tail off this week. He’s worked so, so hard and we did a mock trial, pre-game workout and he failed. It didn’t work,” McKeon said.

Shaq had been bothered by an Achilles injury and came back after missing 27 games. He played five minutes April 3 before he injured his calf.